The most important source of antimony is ores containing the mineral stibnite, antimony trisulfide (Sb.sub.2 S.sub.3). In deposits where stibnite has been exposed to oxidation, a number of oxide minerals may be formed; these include stibiconite (Sb.sub.3 O.sub.6 (OH)), cervantite (Sb.sub.2 O.sub.4 or Sb.sub.2 O.sub.3 -- Sb.sub.2 O.sub.5), valentenite (Sb.sub.2 O.sub.3), senarmonite (Sb.sub.2 O.sub.3), and kermasite (2 Sb.sub.2 S.sub.3 -- Sb.sub.2 O.sub.3) an oxysulfide. Occasionally, native metallic antimony is also found associated with these deposits.
Conventionally, metallic antimony is recovered from these materials or from concentrates prepared therefrom by iron precipitation, direct smelting, or by smelting of oxides formed by roasting thereof. The choice of the pyrometallurgical process steps selected is dictated by the characteristics and quality of the feedstock available and the product(s) desired. The application of any of these pyrometallurgical processes in the production of metallic antimony or high grade antimony oxide results in atmospheric pollution and substantial direct loss of contained antimony.
Pollutants introduced into the atmosphere include suspended paticulates, volatilized antimony trioxide, and gaseous oxides of sulfur. Of these air contaminants, it has been found that the sulfur oxides are the most difficult to control. Meeting existing and proposed air quality control regulations and standards, therefore, is becoming increasingly difficult. The process disclosed herein involves the production of no gaseous discharge stream; hence none of the above-enumerated problems are encountered or involved. The enhanced recovery of the antimony from the processed ore and the elimination of atmospheric-pollutants are readily apparent advantages of this process.
Additionally, substantial losses of antimony content in solid residues, such as liquation residues and slags, of pyrometallurgical processes is generally encountered when these techniques are practiced. Laboratory data obtained for reactions in the process herein disclosed indicate that recoveries between 95 and 100 percent of the contained antimony content in the feedstock are reasonable and practical.
Various attempts have been made in the prior art to devise a successful commercial hydrometallurgical process for producing metallic antimony.
While the desirable characteristics of an economically feasible hydrometallurgical process have long been recognized, the successful development of a commercial process has eluded the prior at. Attempts at developing a commercial process utilizing a ferric chloride as a lixiviant for antimony are disclosed in Bonneville, British Pat. No. 2203 (1870); Butterfield, British Pat. No. 9052 (1896); and Tugov, "Hydrometallurgical Method for Obtaining Metallic Antimony from Concentrates," International Chemical Engineering, V: 1, pp. 5 - 8 (January, 1965).
The Butterfield patent and the Tugov article were expressly concerned with recovery of metallic antimony, but the methods disclosed in both references are unsatisfactory commercially because of their requirement of the use of scrap iron to precipitate the metallic antimony from the antimony chloride solution. This requirement, with the waste products attendant to antimony precipitation with scrap iron, makes these previously described processes commercially impractical and undesirable.
Holmes, U.S. Pat. No. 2,331,395 (1943), discloses an electrolytic hydrometallurgical process for the production of metallic antimony. However, the Holmes process requires the systematic addition of caustic soda (sodium hydroxide) to the process, and produces certain barium salts as an undesirable by-product (which are regenerated as a necessary reactant by a heating process); whereas the process disclosed herein completely regenerates its solutions for some antimony-containing materials, and produces elemental sulfur (which may be removed and sold) as its by-product. Further more, the Holmes process is based upon an alkaline sulfide leaching solution (particularly sodium sulfide), whereas the process disclosed herein is based upon a ferric chloride leaching solution.